1. Field of the Invention
This invention relates to an improved pulping process which utilizes nonionic and anionic surfactants as solubilizing agents to enhance white liquor penetration into wood chips and the like during chemical pulping.
2. Description of the Related Art
Chemical pulping is a process whereby wood chips, wood shavings, and/or sawdust are heated at elevated temperatures in an aqueous acid or alkaline solution, also known as white liquor or cooking liquor, in order to remove enough lignin so that the cellulose fibers can be readily separated from one another. Typically, the process is carried out by heating a mixture of wood chips and cooking liquor in a large pressure vessel called a digester. The cooking temperature is usually in the 170-175.degree. C. range with a corresponding cooking time of 90 minutes. The cooked chips are discharged or blown from the digester under pressure, the mechanical force of which breaks up the wood chips into individual fibers, producing the pulp. The pulp from the digester contains fiber and exhausted liquor which is black in color. The black liquor is washed from the pulp which is then screened to remove uncooked chips and other large fragments and sent on for further processing.
The efficiency of the pulping process is reflected in the degree of delignification which depends upon the extent of the penetration of the cooking liquor and the uniformity of the distribution of the liquor within the chips. Inadequate impregnation usually results in a high level of screen rejects and low pulp yield. The current trends in research and development of the pulping industry are leading towards the use of digester aids. Digester aids are materials that are added to the white liquor to increase the yield and rate. To be most efficient, these digester aids must be soluble and stable under the pulping conditions.
Anthraquinone is an example of a compound that is widely employed as a digester aid because of its relatively low cost and lack of interference with downstream paper making operations.
Unfortunately, the known digester aids are not completely satisfactory, for example, for environmental considerations in certain cases or for lack of adequate penetration and extraction of undesirable organic components in other cases. Despite numerous prior attempts, there exists no known system which enhances the efficiency of the pulp digestion to desired levels while meeting other important criteria. It is therefore a principal object of the present invention to substantially enhance the rate of digestion of wood chips and thereby reduce the pulping cycle times in the production of pulp for the paper making process.